Over the years, various electronic timing systems, clocks, or clocking circuits for electronic systems have been developed. Clocks often use a crystal oscillator, e.g., a quartz-crystal resonator, for frequency stability. The very high stiffness and elasticity of piezoelectric quartz make it possible to produce resonators extending from approximately 1 KHz to 200 MHz. Clocks using a crystal oscillator, for example, have been developed which operate at low power and maintain good accuracy at low cost. The disadvantage of these clocks, however, is that they can maintain their timing accuracy only over a narrow temperature range. Outside this narrow temperature range, the frequency variation becomes quite large and the timing error increases considerably. Some of these timing inaccuracies, for example, can be attributed to the inadequate performance of the crystal oscillator.
The performance characteristics of a crystal oscillator, e.g., a quartz-crystal resonator, generally depend on both the particular cut and the mode of vibration. Each "cut-mode" combination is considered as a separate piezoelectric element, and the more commonly used elements often are designated with letter symbols. The temperature coefficient of the frequency of the crystal varies with different cuts, i.e., with the crystal dimensions, and, generally, a parabolic frequency variation with temperature can be observed.
In order to improve the frequency accuracies of clocks, some clocks have also been developed which use a high precision crystal oscillator with a better temperature coefficient, such as a temperature compensated crystal oscillator ("TCXO"). The TCXO requires a temperature sensor and a more accurate crystal. These clocks, however, have the disadvantages of requiring considerably more power, size, and weight than the original simple clock. Also, these clocks are generally more expensive due to the complicated design and the high cost of the special crystal.
Another conventional approach for a clock is to use two crystals. Instead of using a high precision crystal oscillator and a temperature sensor to measure the temperature (e.g., a TXCO), a very temperature stable high frequency crystal or oscillator is used in this approach as a reference frequency. The high frequency crystal has good performance characteristics over the operating temperature range. In other words, the frequency change versus temperature variation is a relatively flat line instead of a parabolic curve. This high frequency crystal can be used to generate a reference frequency, for example, every 10 minutes. Meanwhile, another normal crystal, e.g., 32 KHz, of the clock also is always operating or running and requires only a low level of current. The normal crystal operates in a dual mode by turning one of the load capacitors on and off. This means that the crystal either has a fast frequency by about 75 parts per million ("ppm") or a slow frequency by 35 ppm. By comparing the 32 KHz frequency with the reference frequency every 10 minutes, the 32 KHz frequency can be adjusted automatically by selecting the dual mode operating time. Nevertheless, a clock using this approach is expensive and can be complex.